The present invention relates to an improved method for maximizing throughput for a part being processed in a conveyorized thermal processor.
Thermal processing involves a series of procedures by which an item is exposed to a temperature-controlled environment, and is used in a variety of manufacturing procedures such as heat treating, quenching and refrigerated storage. One example of a thermal processor is a reflow oven. The production of various goods such as electronic circuit boards in solder reflow ovens frequently entails carefully controlled exposure to heating and/or cooling for specific periods of time. The elevated temperature conditions needed to solder component leads onto printed circuit boards must be gradually and uniformly applied to minimize thermal expansion stresses. For this reason, convection heat transfer may be employed in these solder xe2x80x9creflowxe2x80x9d operations. The connecting solder paste incorporates an amalgam of substances that must undergo phase changes at separate temperature levels. Solder reflow is performed by sequentially passing a part (such as a printed circuit board product) through a series of thermally isolated adjacent regions or xe2x80x9czonesxe2x80x9d in the reflow oven, the temperature of each being independently controlled.
Convection heat transfer chambers or zones are typically set to a fixed control temperature throughout a thermal process. A zone may have one or more controlled thermal elements, these each having at least one corresponding control monitoring location. A thermal element may be defined as either a heat source for heating or a heat sink for cooling and is commanded to a control temperature. The temperatures commanded at the control monitoring locations form a xe2x80x9ccontrol temperature profilexe2x80x9d along the reflow oven. The temperature exposure of the part is governed by the processor temperature of the air in each zone and the exposure time within each region. The temperature of the air along the zones forms a xe2x80x9cprocessor temperature profilexe2x80x9d. The series of instantaneous part temperature values as the part travels along the conveyor and through the oven may be called a xe2x80x9cpart temperature profilexe2x80x9d and if based on measured data may be called a xe2x80x9cmeasured part temperature profilexe2x80x9d. The temperature response of the part must satisfy a manufacturer""s specification requirements, which include allowable tolerance bands or tolerance limits around target values. A measured value within the corresponding tolerance limit satisfies the specification. The procedure for operating the oven to obtain temperature data (used in creating a measured part temperature profile) may be called a xe2x80x9ctest processxe2x80x9d.
The temperature response of the part may be monitored by instrumenting the part or adjacent device with one or more thermocouples (or other temperature measuring contact devices such as thermisters or resistance temperature detectors) prior to sending the part into the reflow oven or by remote observation with a thermal sensor. Alternatively, the temperature response of the part may be measured by a remote means such as an infrared or optical scanner. The thermocouple measurements can be sent to a data acquisition device through an attached cable or by a radio transmitter or by similar means. The temperature adjacent the part (for example, along the conveyor) may also be measured by different means. Two examples are a probe having at least one thermocouple conveyed so as to move along with the part, and a fixed probe extending along the length of the oven and positioned adjacent to the conveyor having a plurality of thermocouples disposed along the probe interior.
A thermal processor, such as a reflow oven, may be modulated by a series of n control parameters labeled Cj numbering from j=1, . . . , n. These control parameters may include the oven setpoint temperature at each zone, the conveyor speed, or a combination of these and other variables subject to direct adjustment or indirect influence during the thermal processor operation. Other physical influences on the thermal process include initial conditions, which may depend on the ambient temperature and humidity, as well as characteristics difficult to measure directly, such as convection rate.
A side view diagram of a reflow oven is shown in FIG. 1 as an example of a thermal processor. The reflow oven 10 may have a conveyor 12 aligned along the length of the oven 10 that moves in the direction towards the right from entrance 14 to exit 16. The oven interior may be divided into two or more zones for thermal processing. In the illustration, first and second zones 18a and 18b are shown. Each zone has at least one heating and/or cooling element 20a and 20b and may feature one or more monitoring instruments 22a and 22b in proximity to the elements to monitor thermal processing. These element monitoring instruments 22a and 22b may be thermocouples or thermostats. The oven 10 may also include one or more recirculation fans 24 to increase convection. The conveyor 12 is moved by means of a conveyor motor 26a; the fan is rotated by means of a fan motor 26b. The settings for heating and/or cooling elements 20a and 20b, and the motors 26a and 26b are controlled by a control station 28, which receives settings input from a receiver 30 instructed by an operator 32 or other means. Each control parameter is commanded to a target condition at a control interface between the input receiver 30 and the control parameter. The example shown in FIG. 1 features a first zone interface 34a, a second zone interface 34b, a conveyor interface 34c and a fan control 34d. These control parameters may be expressed as C values in a series of n dimensions where each control parameter is identified as one of C1, C2, . . . , Cn or as Cj where j=1, . . . , n and may be identified as a control series. In the example shown in FIG. 1, n equals four. Measured data from instruments monitoring control performance such as element monitoring instruments 22a and 22b may be received by a data acquisition device 36a and recorded on a storage medium 36b. 
A part 38, such as a printed circuit board, may be placed on the conveyor 12 upstream of the oven entry 14 to be transported through the oven 10 and egressing through the exit 16. The time-varying thermal exposure may be obtained by using a series of adjacent first and second zones 18a and 18b at a conveyor speed such that part location in the oven 10 may be defined by conveyor speed multiplied by time since entry. The Temperature of the part 38 may be monitored remotely by an infrared or optical scanner or else measured conductively by one or more attached thermal sensors such as a thermocouple 40. The measured part temperature data from the thermocouple 40 may be transmitted to the data acquisition device 36a, either by direct connection or broadcast signal, and recorded on storage medium 36b. 
The measured part temperature data by thermocouple 40 may be compared to the specification ranges to determine whether the control commanded values from the control station 28 produce a part temperature that complies with specification. The specification ranges represent the allowable limits for calculated feedback parameters that are selected to characterize the thermal process for the part. These feedback parameters may be written as B values in a series of m dimensions where each feedback parameter is identified as one of B1, B2, . . . Bm, or as Bi where i=1, . . . , m. The amount by which a measured value deviates from the middle of its specification range corresponds to a feedback index, and the maximum of these feedback indices denotes the Process Window Index, S, for that thermal process.
Temperature variation may be compared along the reflow oven""s length between the commanded temperatures for the elements, the measured zone temperatures, the measured temperature response for the part and the corresponding specification range for the part. A graph illustrating temperature along the conveyor path is shown in FIG. 2. The temperature scale forming the ordinate 50 is plotted against the abscissa 52 or distance axis along the conveyor path. The first zone 54a and second zone 54b encompasses a spatial region across a portion of the oven distance 52. The control temperature levels for each zone region can be appended to form a control profile 56 that is piece-wise continuous along the distance axis 52. Processor temperatures measured (or otherwise determined) near the thermal elements form a processor profile 58. The processor temperature in processor profile 58 often deviates from the control temperature in control profile 56 by a small quantity under convective heat transfer due to cross flow between zones and other physical effects.
The part exhibits a temperature response based on temperature measurements over time along the oven length. This response may be plotted as a part profile 60 along the distance axis 52 by multiplying the conveyor speed by the time from the part""s entry into the oven. Air (the heat transfer medium in the oven) has a relatively low thermal conductivity (compared to the part) with which to convect heat from thermal elements to the part surface. Due to the low convection and the part""s internal heat capacity by virtue of its mass, the part response temperature along part profile 60 will lag the control temperature 56 set in the zone region to which the part is exposed.
The part temperature profile 60 may be evaluated for particular characteristics earlier referred to as feedback parameters related to the thermal process specification. For example, the maximum temperature rise rate 62 may be determined from a discrete temperature increase 62a over a selected time interval 62b, preferably when the temperature increases relatively steadily for the selected time interval 62b. Additionally, the peak part temperature 64 may be determined by the highest measured temperature along the part temperature profile 60. Similarly, xe2x80x9ctime above reflowxe2x80x9d may be ascertained as a reflow time interval 66 by establishing the time period when the part temperature profile 60 exceeds the reflow temperature level 66a beginning at the initial time 66b and ending at final time 66c. These feedback parameter values may be compared to specified ranges that the profile must be within in order to have been properly thermally processed. The maximum temperature rise rate 62 may be compared to the rise rate range, featuring a minimum acceptable rate 68a and a maximum acceptable range 68b. The peak part temperature 64 may be compared to the peak range bounded by a minimum accepted peak temperature 70a and a maximum accepted temperature 70b. Similarly, the reflow time interval 66 may be compared to the reflow time range, with a minimum accepted period 72a and a maximum accepted period 72b. The closer the feedback parameters conform to the middle of the ranges, the smaller its feedback index, the maximum of which yields the Process Window Index, described in more detail below.
Typically in the past, the test procedure for a reflow thermal process involved having an operator set the oven controls, allow sufficient time for the processor temperatures in each zone to reach thermal equilibrium, set the conveyor speed, and send an instrumented part through the oven. Thermal equilibrium is defined as a steady-state condition in which the temperature has stabilized and does not change with time. In practice, a small fluctuation within a specified range is allowed. After a comparison between the part""s target temperatures and its measured values along the oven length, the operator guesses or estimates changes to the control parameters or the conveyor speed. The operator implements the guess and repeats the process over and over until the temperature difference between target and measurement is reduced to an allowable tolerance. An allowable tolerance constitutes an acceptable deviation from the target temperature profile for the part. The trial and error method can be time consuming and requires an experienced operator to implement.
In an improvement to the earlier procedure, guessing the control adjustment may be replaced with a computer executed algorithm that computes a part temperature difference profile between the target and measured values, and uses this difference profile in another algorithm that provides a series of changes to the control parameters in order to bring the measured part temperature profile closer to its target temperature profile. By successive iteration, control parameters to achieve allowable tolerances may be established within two to five attemptsxe2x80x94more quickly than through trial and error. The algorithms used in that method, however, require knowledge of the material properties of the board along with information on the nature of air current within the zone, limiting the method""s practicality and complicating its implementation.
Process Window Index
In an improvement to the earlier procedure, a feedback mechanism may be included which compares measurement-based data against a series of specified tolerances to indicate whether the part temperature responses satisfy the required conditions imposed by the part manufacturer. The numerical sign for whether the required conditions are satisfied may be called the Process Window Index, which represents a nondimensional measure of a part temperature profile to identify whether that profile satisfies the specification imposed by manufacturing requirements. It provides the operator a quantitative indicator of whether the control parameters require adjustment for production-mode thermal processing.
The Process Window Index, S, is a nondimensional positive real number described by a series of measured or measurement-derived parameters Bi numbering from i=1, . . . , m each compared to its tolerance band that the value for Bi must be within to satisfy the required conditions. These measured or measurement-derived parameters, which are hereafter called calculated feedback parameters, form a plurality of data values. One example of a measured parameter for Bi is a peak temperature of the part Tppeak as it moves along the conveyor in the reflow oven. A peak temperature on a part is measured on at least one location on the part, and typically a plurality of temperatures may be obtained at different locations on the part in order to monitor the spatial nonuniformity of the part""s transient response.
An example of a measurement-derived parameter for Bi would be a maximum of part temperature change with respect to time which is called part temperature change rate labeled as ∂Tp/∂t. (The xe2x80x9c∂xe2x80x9d sign represents the partial derivative in differential equations.) Such a quantity is not directly measured, but may be found in discretized form by subtracting part temperature measured at two separate times and dividing by the elapsed time between the measurements. The part temperature change rate is typically monitored to minimize the risks of physical distortion of the part and maximize production throughput, and thus represents an important parameter to monitor. A second example of a measured-derived parameter used in electronics thermal processing applications is the xe2x80x9ctime above reflowxe2x80x9d tar:(Tpxe2x89xa7Tliq) meaning the period during which the part temperature has reached or exceeded the solder liquefaction temperature. This time above reflow may be monitored so as to better enable adequate liquification of the solder for a proper electrical connection between the printed circuit board and its mounted components, without damaging that board from excessive exposure to elevated temperatures.
The tolerance band may be described by its maximum and minimum values. The Process Window Index represents the largest excursion relative to the tolerance bands from the series of calculated feedback parameters. An expression for the Process Window Index S may thus be shown as equation (1):
S=max{|(Bixe2x88x92xcex2+i)|/xcex2xe2x88x92i∀i=1, . . . , m}xe2x80x83xe2x80x83(1)
where xcex2+i=(Bimax+Bimin)/2 or the midpoint within the tolerance band and xcex2xe2x88x92i=(Bimaxxe2x88x92Bimin)/2 or the allowed excursion from the midpoint. (The symbol ∀ means xe2x80x9cfor allxe2x80x9d.) The values Bimax and Bimin represent the maximum and minimum limits within the tolerance band from the target value (typically the midpoint xcex2+i) for the feedback parameters Bi. The Process Window Index is the maximum normalized absolute value of the deviation with respect to the tolerance bands of all feedback parameters, and when displayed is multiplied by one-hundred to provide a percentage value for the amount of tolerance xe2x80x9cwindowxe2x80x9d that is taken in the test process. This represents the typical form by which the Process Window Index is provided to the operator. In order to satisfy the specification requirements, the percentage yielding Process Window Index in this percentage form should have a value of less than one-hundred.
In the procedure to obtain the Process Window Index, calculated feedback parameters are compared in the above-described normalized form, and include parameters such as peak rise rate in the part temperature profile, time period that the part temperature profile is within a specified temperature range, and part temperature profile peak value within the reflow oven. Individual parameters may represent the dominant characteristic in local portions of the reflow oven. For example, part temperature rise rate generally reaches its peak in the initial zones, since the temperature difference between the part initially at room temperature and the first reflow oven zones that the part encounters is often higher than the temperature difference between the partially heated part and the subsequent elevated control temperature zones. The peak value of part temperature may be typically attained towards the end of the reflow process where the control temperature setting is highest.
Examination of the Process Window Index enables an operator to determine whether adjustment of the control parameters for the thermal process may be required. However, the solutions to improve the Process Window Index towards a lower value need not be unique, and so the operator must choose from several options, often based on arbitrary criteria. In addition, when the number of control parameters Cj where j=1, . . . , n exceeds a certain value, such as n greater than 4, the multitude of potential solutions becomes too great to practicably sort for the most appropriate selection.
While the Process Window Index provides feedback by which to adjust control parameters, no method or mechanism is available to provide mathematically efficient correlation between changes in control parameters and effect on feedback parameters to indicate specification compliance for the selected control parameters. Such a development would greatly expedite the production of parts in a thermal processor by eliminating trial and error to adjust the control parameters.
A method for maximizing throughput for thermal processing of a part searches for the maximum conveyor speed while calculating a Process Window Index that is less than a required value. The best Process Window Index associated with the maximum conveyor speed corresponds to a maximum throughput control series with which to set the control parameters of the thermal processor.